Modeling the effects of plant uptake dynamics on nitrogen removal of a bioretention system

Bioretention systems are widely used to remove nutrients from urban runoff. Plants play a key role in the nitrogen removal performance of bioretention systems; however, few model studies have focused on the effects of plant uptake dynamics on the performance. In this study, we propose an eco-hydrological model of bioretention systems by coupling plant, hydrological, and nitrogen modules. The eco-hydrological model was verified using observed data on biomass, plant nitrogen uptake, hydrological performance, and effluent nitrogen concentrations of a bioretention system planted with Canna indica L. in Shenzhen, China. The validated model was used to evaluate the effects of seasonal or interannual variation of plant nitrogen uptake on nitrogen removal performance of the bioretention system. The results indicated that (i) The model is able to explicitly describe plant dynamics and nitrogen transformation processes of the system, and the Nash Sutcliffe Efficiency coefficients of biomass, monthly plant nitrogen uptake, and effluent nitrogen concentrations were all greater than 0.6; (ii) The plant nitrogen uptake is sensitive to most parameters in the plant module; The effluent nitrogen concentrations are sensitive to decomposition constant of plant residues (k_res) during most months of the year and also sensitive to optimal nitrogen content at the germination (bn₁) and optimal nitrogen content at the maturity (bn₂) during the months with low influent nitrogen concentrations; and (iii) The seasonal and interannual variations in plant nitrogen uptake significantly affect nitrogen removal efficiencies, especially during rainy seasons. Therefore, the model can consider the influence of plant nitrogen uptake on the long-term nitrogen removal performance of bioretention systems.